The double fertilization process of flowering plants results in a diploid embryo and a triploid endosperm tissue, which are otherwise genetically identical. Endosperm has 2 n of the maternal genome and 1 n of the paternal genome. It has been shown that such a unique genetic composition is important to normal seed development (Rhoades and Dempsey, Genetics 54:505–22 (1966); Lin, Genetics 107:103–15 (1984); Scott et al., Development 125:3329–41 (1998)).
The parental-origin-specific modification of a gene can cause various effects on the expression of the two parental alleles in the offspring. Among various parental effects, genomic imprinting, in which expression of the gene is dependent on the parental source of its transmission, is the most studied. Imprinting is a developmental phenomenon wherein a gene in a gamete or zygote is modified such that preferential expression of a single parental allele occurs in the offspring. The definition of genomic imprinting does not necessarily require monoallelic expression; instead, both alleles may be expressed but expressed unequally (Feinberg, Curr. Top. Microbiol. Immunol. 249:87–99 (2000)).
Limited information is available at the gene expression level about the differences between the parental genomes, such as transcription activation and regulation of the maternal and paternal genomes after the union of the central and sperm cell and during endosperm development. To date, only a few genes demonstrating imprinting effects have been found in plants. Some of the imprinted genes include R, a transcription factor involved in the anthocyanin pigment pathway of maize; dzr1, which conditions low accumulation of the 10-kDa zein in maize endosperm; MEA and FIE, similar to Drosophila polycomb proteins; and FIS2, a DNA-binding transcriptional regulatory protein in Arabidopsis (Kermicle, Genetics 66:69–85 (1970); Chaudhuri and Messing, Proc. Natl. Acad. Sci. USA 91:4867–71 (1994); Grossniklaus et al., Science 280:235–41 (1998); Luo et al., Proc. Natl. Acad. Sci USA 96:296–301 (1999); and Luo et al., Proc. Natl. Acad. Sci USA 97:10637–42 (2000)). All of the imprinted genes identified in plants to date involved inactivation of the paternal allele and were associated with endosperm tissue, although there are inconsistencies in reporting whether MEA affects embryo tissue (Kinoshita et al., The Plant Cell 11:1945–52 (1999)).
In contrast to plants, genes that are expressed from only one of the parental alleles have been well characterized in mammals, where the disturbance of imprinting can result in dramatic developmental aberrations and cancer (Reik and Maher, Trends Genet 13(8):330–4 (1997)). In this taxonomic group, approximately 20 genes have been identified as imprinted genes (Bartolomei and Tilghman, Ann. Rev. Genet 31:439–525 (1997)). Many of these imprinted genes appear to regulate the expression of developmentally important genes. A recent study reported the parent-of-origin effect on quantitative trait loci (QTLs) for mouse body composition (De Koning et al., Proc. Natl. Acad. Sci. USA 97:7947–50 (2000). Four out of five QTLs detected were found subject to imprinting, indicating that genomic imprinting might be a more common phenomenon than previously thought, even for complex traits.
In view of the limited information about imprinted genes in plants, in particular their role in endosperm development, it would be desirable to identify and characterize such genes. Accordingly, the inventors have identified a novel gene in maize endosperm that is imprinted and which shows homology to the Petunia No-Apical Meristem (NAM) gene. NAM (no-apical meristem) has been shown to be required for pattern formation in embryos and flowers, and Petunia embryos carrying the NAM mutation fail to develop a shoot apical meristem (Souer et al., Cell 85(2):159–70 (1996)). Shoot apical meristem is a collection of undifferentiated cells set aside during embryogenesis. The production of vegetative structures, such as leaves or shoots, and of reproductive structures, such as flowers, is temporally segregated, such that a leaf or shoot arises early in a plant life cycle, while a flower develops later. The transition from vegetative to reproductive development is the consequence of a process termed floral induction (Yanofsky, Ann. Rev. Plant Physiol. Plant Mol. Biol. 46:167–88 (1995)).
Once induced, shoot apical meristem either persists and produces floral meristem, which gives rise to flowers, and lateral meristem, which gives rise to branches, or is itself converted to floral meristem. Floral meristem differentiates into a single flower having a fixed number of floral organs in a whorled arrangement. Dicots, for example, contain four whorls (concentric rings), in which sepals (first whorl) and petals (second whorl) surround stamens (third whorl) and carpels (fourth whorl).
Although shoot meristem and floral meristem both consist of meristemic tissue, shoot meristem is distinguishable from the more specialized floral meristem. Shoot meristem generally is indeterminate and gives rise to an unspecified number of floral and lateral meristems. In contrast, floral meristem is determinate and gives rise to the fixed number of floral organs that comprise a flower.
The NAM gene appears to be a member of a large gene family that is suggested to comprise transcriptional factors important to plant development (Souer et al., supra; Kikuchi et al., Mol. Gen. Genet 262(6):1047–51 (2000)).